1. Field of the Invention
The present invention relates to a process for producing a semiconductor layer on a substrate and an apparatus for producing the semiconductor layer, and particularly to process and apparatus for producing a semiconductor layer of a non-monocrystalline silicon type such as amorphous silicon, amorphous silicon germanium, amorphous silicon carbide or microcrystalline silicon, which is used in solar cells, photosensitive drums for copying machines, image sensors for facsimiles, thin film transistors for liquid crystal display devices, etc. The present invention also relates to a process for fabricating a photovoltaic cell using such a production process.
2. Related Background Art
Amorphous silicon permits the formation of a large-area semiconductor film by a plasma CVD process and so permits the fabrication of a large-area semiconductor device with comparative ease compared with crystalline silicon and polycrystalline silicon.
Therefore, amorphous silicon films are often used in semiconductor devices of which a large area is required, specifically, in solar cells, photosensitive drums for copying machines, image sensors for facsimiles, thin film transistors for liquid crystal display devices, etc.
These devices are larger in the area of a device compared with devices comprising crystalline semiconductors, such as LSI and CCD. In the case of, for example, a solar cell, an area as large as about 30 m2 per home is required to provide an output of about 3 kW, by which electric power for general home is furnished, when a conversion efficiency thereof is 10%. Therefore, a solar cell module also occupies a considerable large area.
The formation of an amorphous silicon film is generally conducted by a plasma CVD process in which a Si-containing raw gas such as SiH4 or Si2H6 is decomposed by high-frequency discharge into a plasma state, and a film is formed on a substrate placed in the plasma.
In the case where the amorphous silicon film is formed by the plasma CVD process, high frequency of RF (radio frequency; about 13.56 MHz) has heretofore been generally used.
On the other hand, attention has been paid to plasma CVD using VHF (very high frequency) in recent years.
For example, Amorphous Silicon Technology, pp. 15-26, 1992 (Materials Research Society Symposium Proceedings Volume 258) reports that discharge frequency is changed from RF of 13.56 MHz to VHF, whereby high-density plasma can be obtained to enhance a film-forming rate to a markedly high degree, so that a good deposited film can be formed at a high speed.
U.S. Pat. No. 4,406,765 discloses a high-frequency plasma CVD process in which a direct current (DC) electric field is applied. It is said that a moderate DC electric field is applied together with a high-frequency electric field in the plasma CVD process, whereby a good-quality amorphous semiconductor can be provided.
However, the application of the plasma CVD process using VHF, which is capable of forming a deposited film at a high speed as described above, to deposition of a large-area film has involved the following problems.
Namely, when a large-area flat plate discharge electrode generally used in RF is used to induce uniform discharge over a large area, impedance is hard to be matched in VHF, resulting in difficulty of obtaining uniform plasma on the discharge electrode.
When a rod or radial antenna electrode is used, impedance is matched. However, the balance of an area ratio between the discharge electrode and an opposite electrode, which is substantially 1 in parallel-plate electrodes, is destroyed due to the extremely small area of the discharge electrode. The absolute value of self-bias, which should become smaller in small-area parallel-plate electrodes as frequency increases, becomes greater on the contrary, so that the discharge electrode generates high negative self-bias voltage. In this case, the area of the discharge electrode is small, and so a large-area substrate cannot be place thereon, and high positive voltage against the discharge electrode is applied to the substrate.
As disclosed in U.S. Pat. No. 4,406,765 described above, it is important to apply the moderate DC electric field together with the high-frequency electric field in order to provide a good-quality amorphous semiconductor. It has however been difficult to control the quantity of bias supply power for suitably controlling the DC electric field without causing abnormal discharge such as spark in a discharge chamber or causing dielectric breakdown by charge-up on the surface of the resulting deposited film.
In order to control the DC electric field, it has been known to apply high-frequency bias power in addition to the high-frequency power for decomposing a raw gas by discharge, besides the above-described method by the application of the DC voltage. This method is disclosed in Japanese Patent Application Laid-Open No. 6-232429 and the like. Even in this case, it has been difficult to control the quantity of bias supply power for suitably controlling the DC electric field without causing abnormal discharge such as spark in a discharge chamber or causing dielectric breakdown by charge-up on the surface of the resulting deposited film.
In addition, as a continuous fabrication apparatus of an amorphous silicon type semiconductor device, U.S. Pat. No. 4,400,409 and the like disclose a continuous plasma CVD apparatus using a roll to roll system.
According to this apparatus, plural glow discharge chambers are provided, and a large-area device having semiconductor junctions can be continuously fabricated by arranging the glow discharge chambers along a passageway through which a sufficiently long band-like substrate having a desired width successively passes and feeding the substrate in the longitudinal direction thereof while depositing and forming a semiconductor film of a necessary conductive type in each glow discharge chamber.
As described above, when the continuous plasma CVD apparatus of the roll to roll system is used, the device can be continuously fabricated for a long period of time without stopping the fabrication apparatus, so that high productivity can be achieved.
When DC electric field is applied to plasma in this plasma CVD process of the roll to roll system, however, the same problems as described above have been encountered in VHF in particular.
The roll to roll system has also involved a problem that although there is a plurality of discharge chambers in which a deposited film is formed, DC voltage of different levels cannot be applied to the plural discharge chambers by such a method by applying DC voltage to the substrate side as disclosed in U.S. Pat. No. 4,406,765, since the substrate is continuous and common and is generally conductive, so that a bias voltage level cannot be suitably set according to the kind of a deposited film and discharge conditions in each discharge chamber.
It is an object of the present invention to provide a process and an apparatus which can solve the problem involved in the case where in the abovedescribed process for forming a semiconductor layer, the VHF plasma CVD process capable of achieving a high film-forming rate is applied to the deposition of a large-area film, that is to say, the problem that although it is necessary to apply a moderate DC electric field together with a high-frequency electric field, it is difficult to control the quantity of bias supply power for achieving a good bias effect while preventing the occurrence of a failure by spark or charge-up in the resulting deposited film, so as to easily set a proper quantity of bias supply power, thereby depositing a good-quality semiconductor layer over a large area at a high speed.
Another object of the present invention is to provide a process and an apparatus for producing a semiconductor layer by continuously forming a deposited film over a large area, by which the VHF plasma CVD process capable of obtaining high-density plasma is introduced for forming a plurality of layers, and DC electric fields are controlled to respective proper levels, whereby a laminated good-quality semiconductor film can be produced.
The above objects can be achieved by the present invention described below.
According to the present invention, there is thus provided a process for producing a semiconductor layer by introducing a raw gas into a discharge chamber and supplying high-frequency power to the chamber to decompose the raw gas by discharge, thereby forming a semiconductor layer on a substrate within the discharge chamber, the process comprising the steps of:
supplying high-frequency power of at least very high frequency (VHF) as the high-frequency power;
supplying bias power of direct current power and/or high-frequency power of radio-frequency (RF) together with the high-frequency power of VHF to the discharge chamber; and
controlling a direct current component of an electric current flowing into an electrode, to which the bias power is supplied, so as to fall within a range of from 0.1 A/m2 to 10 A/m2 in terms of a current density based on the area of an inner wall of the discharge chamber.
According to the present invention, there is also provided a process for producing a semiconductor layer by introducing a raw gas into a plurality of discharge chambers, supplying high-frequency power to the chambers to decompose the raw gas by discharge, and causing a substrate to successively pass through the discharge chambers, thereby forming a plurality of semiconductor layers on the substrate, the process comprising the steps of:
supplying high-frequency power of very high frequency (VHF) as the high-frequency power to two or more discharge chambers of the plural discharge chambers;
supplying bias power of different levels from each other to the discharge chambers, to which the high-frequency power of VHF is supplied, according to respective film-forming conditions in the discharge chambers; and
controlling the electric potential of each electrode, to which the bias power is supplied, to the same level as that of the substrate or positive potential against the substrate.
According to the present invention, there is further provided a process for fabricating a photovoltaic cell, comprising the steps of introducing a raw gas into a discharge chamber and supplying high-frequency power to the chamber to decompose the raw gas by discharge, thereby forming an i-type semiconductor layer on a substrate within the discharge chamber, the process comprising the steps of:
supplying high-frequency power of at least very high frequency (VHF) as the high-frequency power in the step of forming the i-type semiconductor layer;
supplying bias power of direct current power and/or high-frequency power of radio-frequency (RF) together with the high-frequency power of VHF to the discharge chamber; and
controlling a direct current component of an electric current flowing into an electrode, to which the bias power is supplied, so as to fall within a range of from 0.1 A/m2 to 10 A/m2 in terms of a current density based on the area of an inner wall of the discharge chamber.
According to the present invention, there is still further provided a process for fabricating a photovoltaic cell by introducing a raw gas into a plurality of discharge chambers, supplying high-frequency power to the chambers to decompose the raw gas by discharge, and causing a substrate to successively pass through the discharge chambers, thereby at least forming a plurality of i-type semiconductor layers on the substrate, the process comprising the steps of:
supplying high-frequency power of very high frequency (VHF) as the high-frequency power to two or more discharge chambers of the plural discharge chambers in which the respective i-type semiconductor layers are formed;
supplying bias power of different levels from each other to the discharge chambers, to which the high-frequency power of VHF is supplied, according to respective film-forming conditions in the discharge chambers; and
controlling the electric potential of each electrode, to which the bias power is supplied, to the same level as that of the substrate or positive potential against the substrate.
The above-described production and fabrication processes may be used in combination.
In each process, a raw gas comprising a silicon atom-containing molecule may preferably be used as the raw gas to form a silicon type non-monocrystalline semiconductor layer on the substrate.
The substrate and the inner wall surface of the discharge chamber may preferably be controlled to earth potential.
The bias power may preferably be supplied to an electrode provided independently of the electrode to which the high-frequency power of VHF is supplied, or to the electrode to which the high-frequency power of VHF is supplied. DC power may be preferably used as the bias power.
The semiconductor layer may preferably be formed in accordance with a plasma CVD process.
A band-like and/or conductive substrate may preferably be used as the substrate.
The substrate may preferably be used as a part of the inner wall of the discharge chamber.
According to the present invention, there is yet still further provided an apparatus for producing a semiconductor layer by introducing a raw gas into a discharge chamber and supplying high-frequency power to the chamber to decompose the raw gas by discharge, thereby forming a semiconductor layer on a substrate within the discharge chamber, the apparatus comprising:
a means for supplying high-frequency power of at least very high frequency (VHF) as the high-frequency power;
a means for supplying bias power of direct current power and/or high-frequency power of radio-frequency (RF) together with the high-frequency power of VHF to the discharge chamber; and
a means for controlling a direct current component of an electric current flowing into an electrode, to which the bias power is supplied, so as to fall within a range of from 0.1 A/m2 to 10 A/m2 in terms of a current density based on the area of an inner wall of the discharge chamber.
According to the present invention, there is yet still further provided an apparatus for producing a semiconductor layer by introducing a raw gas into a plurality of discharge chambers, supplying high-frequency power to the chambers to decompose the raw gas by discharge, and causing a substrate to successively pass through the discharge chambers, thereby forming a plurality of semiconductor layers on the substrate, the apparatus comprising:
a means for supplying high-frequency power of very high frequency (VHF) as the high-frequency power to two or more discharge chambers of the plural discharge chambers;
a means for supplying bias power of different levels from each other to the discharge chambers, to which the high-frequency power of VHF is supplied, according to respective film-forming conditions in the discharge chambers; and
a means for controlling the electric potential of each electrode, to which the bias power is supplied, to the same level as that of the substrate or positive potential against the substrate.
In these apparatus, the means for supplying the high-frequency power of VHF may preferably comprise a discharge electrode and a high-frequency power source of VHF connected to the discharge electrode. The means for supplying the bias power may preferably comprise a bias electrode provided separately from the discharge electrode and a power source connected to the bias electrode, or comprise a power source connected to the discharge electrode.
In the case where the means for supplying the bias power comprises a direct current power source connected to the discharge electrode, the power source may preferably be connected through a high-frequency power blocking means. At that time, the high-frequency power source may preferably be connected to the discharge electrode through a direct current power blocking means.